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Original Article
ECG Voltage in Relation to Peripheral and Central
Ambulatory Blood Pressure
Wen-Yi Yang,1 Blerim Mujaj,1 Ljupcho Efremov,1 Zhen-Yu Zhang,1 Lutgarde Thijs,1
Fang-Fei Wei,1 Qi-Fang Huang,1 Aernout Luttun,2 Peter Verhamme,2 Tim S. Nawrot,3
José Boggia,4 and Jan A. Staessen1,5
BACKGROUND
The heart ejects in the central elastic arteries. No previous study in
workers described the diurnal profile of central blood pressure (BP) or
addressed the question whether electrocardiogram (ECG) indexes are
more closely associated with central than peripheral BP.
METHODS
In 177 men (mean age, 29.1 years), we compared the associations of
ECG indexes with brachial and central ambulatory BP, measured over 24
hours by the validated oscillometric Mobil-O-Graph 24h PWA monitor.
RESULTS
From wakefulness to sleep, as documented by diaries, systolic/diastolic BP decreased by 11.7/13.1 mm Hg peripherally and 9.3/13.6 mm
Hg centrally, whereas central pulse pressure (PP) increased by 4.3 mm
Hg (P < 0.0001). Over 24 hours and the awake and asleep periods, the
peripheral-minus-central differences in systolic/diastolic BPs averaged
11.8/–1.6, 12.7/–1.8, and 10.3/–1.2 mm Hg, respectively (P < 0.0001).
Cornell voltage and index averaged 1.18 mV and 114.8 mV·ms. Per 1-SD
increment in systolic/diastolic BP, the Cornell voltages were 0.104/0.086
mV and 0.082/0.105 mV higher in relation to brachial 24-hour and
asleep BP and 0.088/0.90 mV and 0.087/0.107 mV higher in relation to
central BP. The corresponding estimates for the Cornell indexes were
9.6/8.6 and 8.2/10.5 mV·ms peripherally and 8.6/8.9 and 8.8/10.7 mV·ms
centrally. The regression slopes (P ≥ 0.067) and correlation coefficients
(P ≥ 0.088) were similar for brachial and central BP. Associations of ECG
measurements with awake BP and PP were not significant.
CONCLUSIONS
Peripheral and central BPs run in parallel throughout the day and are
similarly associated with the Cornell voltage and index.
Keywords: ambulatory blood pressure; blood pressure; blood pressure monitoring; central blood pressure; clinical science; ECG voltage;
hypertension.
doi:10.1093/ajh/hpx157
Ambulatory blood pressure (BP) monitoring provides information not only on the BP level but on the diurnal BP profile as
well. The Mobil-O-Graph 24h PWA Monitor (I.E.M. GmbH,
Stolberg, Germany) is a portable monitor validated for the
recording of brachial BP.1 It includes the ARCSolver software,2
which allows estimating central BP. We3 and other researchers4
validated the central hemodynamic measurements in resting
conditions against a tonometric3 or invasive standard.4
The heart ejects blood directly into the central elastic
arteries. Compared with conventional brachial pressure,
several5 but not all6 studies suggest that central pressure
is more strongly related to target organ damage and the
incidence of cardiovascular complications.7,8 In view of
the close anatomical proximity of central arteries to the
heart and the strong association of electrocardiogram
(ECG) voltages with BP,9 we considered that relating ECG
Correspondence: Jan A. Staessen (jan.staessen@med.kuleuven.be).
1Studies Coordinating Centre, Research Unit Hypertension and
Cardiovascular Epidemiology, KU Leuven Department of Cardiovascular
Sciences, Faculty of Medicine, University of Leuven, Leuven, Belgium;
2Centre for Molecular and Vascular Biology, KU Leuven Department
of Cardiovascular Sciences, Faculty of Medicine, University of Leuven,
Leuven, Belgium; 3Centre for Environmental Sciences, Hasselt
University, Diepenbeek, Belgium; 4Unidad de Hipertensión Arterial,
Departamento de Fisiopatología, Centro de Nefrología, Hospital de
Clínicas, Universidad de la República, Montevideo, Uruguay; 5R & D VitaK
Group, Maastricht University, Maastricht, The Netherlands.
Initially submitted August 2, 2017; date of first revision August 19, 2017;
accepted for publication September 8, 2017
© The Author 2017. Published by Oxford University Press on behalf
of American Journal of Hypertension, Ltd.
This is an Open Access article distributed under the terms of the Creative
Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/), which permits non-commercial re-use,
distribution, and reproduction in any medium, provided the original work
is properly cited. For commercial re-use, please contact journals.permissions@oup.com
American Journal of Hypertension 1
Yang et al.
voltages to peripheral and central BP might generate new
insights. In fact, no previous study in workers described
the diurnal profile of central BP or addressed the question
whether ECG voltages are more closely associated with
central than peripheral ambulatory BP. We addressed these
issues in male workers enrolled in the Study for Promotion
of Health in Recycling Lead (SPHERL [NCT02243904]).10
METHODS
Study population
SPHERL complies with the Helsinki declaration.11 The
Ethics Committee of the University Hospitals Leuven
approved the study. A detailed protocol has been published
elsewhere.10 In short, the nursing staff at lead acid battery manufacturing and recycling plants in the United States enrolled
new hires for detailed health evaluations prior to blood lead
elevations associated with occupational exposure to lead. Of
490 men joining the work force and invited to participate,
336 provided informed written consent (participation rate,
68.6%). Of those, 284 had both their ECG and 24-hour ambulatory BP recorded. We excluded 107 participants, because the
central 24-hour ambulatory BP did not meet quality criteria
(see below; n = 100), because their ECG did not include the
precordial leads (n = 4), or because they had incomplete or
complete right or left bundle branch block (n = 3). Thus, the
number of men statistically analyzed totaled 177.
Electrocardiography
We used the Cardiax device (RDSM Medical Devices,
Hasselt, Belgium) to record 12-lead ECGs at a speed of 25 mm/s
with the calibration set at 1 mV/cm. Voltages and QRS duration
were measured to the nearest 0.1 mV and 1 ms, respectively.
Low-frequency noise originating from movement, baseline
wander, and respiration and high-frequency noise emanating
from power-lines or radiated electromagnetic influence were
filtered before the final signal acquisition. In accordance with
the recommendations of the American Heart Association,12
cutoff values were set at 0.05 Hz and 150 Hz for the low- and
high-frequency filters, respectively. The Cornell voltage was the
sum of the S wave in precordial lead V3 and the R wave in limb
aVL.13 The Cornell index was Cornell voltage multiplied by the
QRS duration from its earliest onset to its latest ending across the
12 ECG leads.14 Cornell voltage and index have high reproducibility and specificity to detect left ventricular hypertrophy15,16
and are therefore recommended to be used in studies of BP.17
The Cardiax software allows exporting all ECG measurements
into an Excel workbook, which was subsequently imported
into SAS, using standardized programming statements, thereby
excluding observer-introduced bias. Two certified cardiologists
(W.-Y.Y. and Z.-Y.Z.) checked the performance of the programming against a visual read of 20 Cornell indexes below the 5th
or above the 95th percentile of their distributions.
Ambulatory measurements
We programmed oscillometric Mobil-O-Graph 24h PWA
monitors (I.E.M. GmbH)1 fitted with the appropriate cuff size
to obtain readings at intervals of 15 and 30 minutes during
2 American Journal of Hypertension
the awake and asleep periods of the day. These intervals were
determined from the diary completed by the workers during
ambulatory monitoring, which was carried out on normal
working days. If the ambulatory monitoring lasted over 1 day,
only the recordings during the first 24 hours were analyzed.
Intraindividual means of the ambulatory measurements were
weighted by the time interval between successive readings.18
The ARCSolver algorithm, as implemented in Mobil-OGraph 24h PWA monitor reconstructs the central pulse wave
by applying a transfer function.2 Recordings of the central
hemodynamics are carried out at the diastolic BP level (±5 mm
Hg) for approximately 10 seconds, using a high-fidelity pressure sensor (MPX5050, Freescale, Tempe, AZ). The transfer
function implemented in the ARCSolver software includes an
algorithm for checking quality of the signal on a scale from 1 to
4. Results of excellent or good quality are labelled 1 and 2 and
respectively include over 80% or over 50% of the cardiac cycles
during signal acquisition. Grade 3 results are estimated from less
than 50% of the recorded cycles and are of poor quality. Grade
4 indicates missing results, because of insufficient signal quality. We included central hemodynamic measurements in the
analyses only if graded 1 or 2. In addition, the ARCSolver software excludes central hemodynamic measurements obtained
at a cuff pressure that is not within 5 mm Hg of diastolic BP.
As mentioned before, the aforementioned quality standards for
central BP eliminated 100 participants from analysis.
Other measurements
Trained nurses measured the participants’ anthropometric characteristics and office BP. They administered a questionnaire to collect information about each worker’s medical
history, smoking and drinking habits, and intake of medications. Office BP was the average of 5 consecutive readings
measured after participants had rested in the sitting position
for at least 5 minutes.19 Standard cuffs had a 12 × 24 cm inflatable bladder, but, if upper arm girth exceeded 31 cm, larger
cuffs with a 15 × 35 cm bladder were used. Office hypertension was a BP of at least 140 mm Hg systolic or 90 mm
Hg diastolic. The corresponding thresholds for the 24-hour
brachial BP were 130 mm Hg and 80 mm Hg. Patients on
antihypertensive drug treatment were categorized as hypertensive irrespective of type of BP measurement. Skinfold
thickness was the average of measurements obtained at 3
sites, the triceps, subscapular, and supra-iliac area by means
of the Harpenden Skinfold Caliper (Bedfordshire, UK) providing a constant pressure of 0.01 kg per mm2 (0.098 N/
mm2) at all openings of the 90 mm2 anvils.
Plasma glucose and serum total and high-density lipoprotein were measured on venous blood samples obtained after
8 hours of fasting. Diabetes mellitus was a self-reported diagnosis, a fasting plasma glucose of 7.0 mmol/l (126 mg/dl) or
higher,20 or use of antidiabetic drugs. We estimated glomerular
filtration rate from serum cystatin C, using the Chronic Kidney
Disease Epidemiology Collaboration cystatin C equation.21
Statistical analysis
For statistical analysis and database management, we
used SAS software, version 9.4 (SAS Institute, Cary, NC). We
ECG Voltage and Peripheral and Central BP
compared means and proportions, using a t-test for paired or
unpaired observations, as appropriate, and the χ2-statistic,
respectively. Statistical significance was an α-level of 0.05.
From the workers’ diaries, we identified the awake and
sleeping periods. Next, we plotted 2-hourly averages of the
ambulatory measurements of central BP level and the heart
rate over 24 hours. Two criteria were applied to differentiate
a significant diurnal rhythm from random variability. First,
in all participants combined, we compared mean awake and
asleep BP levels and heart rate. Second, in individual participants, we used the runs test with a one-sided probability of
5%.22 We used linear regression to assess the relation of the
ECG variables with peripheral and central BPs. We compared
the regression slopes relating the ECG indexes to peripheral
and central BP, using the TEST statement, as implemented in
the PROC REG procedure of the SAS package. To check that
collinearity between peripheral and central BPs did not bias
our results, we applied two approaches. First, we calculated
the residual of central BP removing the contribution to its
variance of peripheral BP, or vice versa, and we introduced
the residual along with the alternative BP in the regression
models. Second, we pairwise compared the correlations of
the ECG indexes with peripheral and central BP, using the
Hotelling–William test.23 Finally, we checked the consistency
of our observations in sensitivity analyses adjusted for body
mass index and skinfolds.
RESULTS
Characteristics of participants
The 177 participants were on average (±SD) 29.1 ± 10.4
years old (5th to 95th percentile interval, 19.1–52.2). The
Cornell voltage averaged 1.18 ± 0.60 mV (5th to 95th percentile interval, 0.29–2.34) and the Cornell index 114.8 ± 60.8
mV·ms (5th to 95th percentile interval, 25.5–230.4). Height
averaged 1.75 ± 0.07 m, weight 86.1 ± 19.7 kg, office systolic/diastolic BP 119.9 ± 9.8/80.5 ± 8.5 mm Hg, the 24-hour
brachial BP 124.7 ± 9.7/73.8 ± 8.1 mm Hg, and total and
high-density lipoprotein cholesterol 4.47 ± 0.96 mmol/l and
1.21 ± 0.28 mmol/l, respectively. No participant had a history of cardiovascular disease, while 75 (32.2%) were smokers, 88 (49.7%) reported regular alcohol intake, 2 (1.1%) had
diabetes mellitus, and 35 (19.8%) had office hypertension, of
whom 11 (31.4%) were on antihypertensive drug treatment.
Table 1 lists the main characteristics of the participants by
the median (107.5 mV·ms) of the Cornell index. The workers with higher index had greater body mass index (P =
Table 1. Characteristics of participants by median ECG Cornell index
Characteristic
Cornell < 107.5 (n = 88)
Cornell ≥ 107.5 (n = 89)
All (n = 177)
Current smoking
28 (31.8)
29 (32.6)
57 (32.2)
Drinking alcohol
46 (52.3)
42 (47.7)
88 (49.7)
Office hypertension
13 (14.8)
22 (24.8)
35 (19.8)
Number (%) of participants
24-Hour ambulatory hypertension
24 (27.3)
42
(47.2)†
66 (37.3)
On antihypertensive treatment
1 (1.1)
10 (11.2)†
11 (6.2)
Diabetes mellitus
0 (0.0)
2 (2.3)
2 (1.1)
Mean (SD) characteristic
Age (year)
27.4 ± 8.8
30.9 ± 11.7
29.1 ± 10.4
Body mass index (kg/m2)
27.0 ± 4.9
29.2 ± 6.0*
28.1 ± 5.6
Skinfolds (cm)
2.26 ± 0.89
2.42 ± 0.95
2.34 ± 0.92
Waist-to-hip ratio
0.96 ± 0.08
0.98 ± 0.08
0.97 ± 0.08
Office blood pressure
Systolic (mm Hg)
117.9 ± 8.5
121.9 ± 10.6†
119.9 ± 9.8
Diastolic (mm Hg)
78.9 ± 8.4
82.1 ± 8.5*
80.5 ± 8.5
Office heart rate (bpm)
73.9 ± 10.5
73.9 ± 11.7
73.9 ± 11.1
3.80 ± 1.05
3.97 ± 1.31
3.88 ± 1.19
Laboratory examination
Total/HDL cholesterol ratio
Cystatin C (mg/l)
eGFR (ml/min/1.73 m2)
0.66 ± 0.10
0.68 ± 0.11
0.67 ± 0.11
131.9 ± 12.4
127.3 ± 15.2*
129.6 ± 14.0
Office hypertension was a blood pressure of ≥140 mm Hg systolic or ≥90 mm Hg diastolic; the corresponding thresholds for the 24-hour
brachial blood pressure were ≥130 mm Hg and ≥80 mm Hg. Patients on antihypertensive drug treatment were categorized as hypertensive
irrespective of type of blood pressure measurement. Diabetes mellitus was a self-reported diagnosis, a fasting plasma glucose of ≥7.0 mmol/l
or use of antidiabetic drugs. eGFR was derived from serum cystatin C, using the Chronic Kidney Disease Epidemiology Collaboration Cystatin
C equation. Significance of the difference between categories: *P ≤ 0.05; †P ≤ 0.01. Abbreviations: ECG, electrocardiogram; eGFR, estimated
glomerular filtration rate; HDL, high-density lipoprotein.
American Journal of Hypertension 3
Yang et al.
0.011) and higher BP (P ≤ 0.020), but lower estimated glomerular filtration rate (P = 0.028). The other characteristics
were similar between the two groups (P ≥ 0.10).
Peripheral and central ambulatory measurements
Number of measurements. For brachial BP, the median
number of readings averaged to estimate mean 24-hour
awake and asleep BP was 35 (interquartile range, 30–43;
5th–95th percentile interval, 21–54), 22 (17–29 and 12–40),
and 11 (7–17 and 5–23). For central BP, the corresponding
numbers were 27 (20–33 and 14–44), 16 (12–24 and 7–35),
and 8 (5–11 and 3–20), respectively.
Levels of central vs. peripheral BP. Over 24 hours (Table
2 and Figure 1), central systolic and pulse pressure (PP) were
11.8 mm Hg (95% confidence interval [CI], 11.3–12.4) and
13.4 mm Hg (CI, 12.9–14.0) (P < 0.0001) lower than the brachial levels. The same was true during the awake and asleep
periods of the recordings with BP differences amounting to
12.7 mm Hg (CI, 12.0–13.3) systolic and 14.4 mm Hg (CI,
13.8–15.1) for PP during waking hours and 10.3 mm Hg (CI,
9.71–10.9) and 11.5 mm Hg (CI, 10.9–12.1), respectively,
during sleep. The average differences of peripheral-minuscentral diastolic BP (P < 0.0001; Table 2) amounted to –1.58
mm Hg (CI, –1.64 to –1.53) over 24 hours, –1.78 mm Hg
(CI, –1.84 to –1.72) awake and –1.23 (CI, –1.32 to –1.13)
asleep.
Levels of awake vs. asleep BP. Peripheral BP (Table 2)
decreased from wakefulness to sleep by 11.7 mm Hg systolic (CI, 10.0–13.4; P < 0.0001) and 13.1 mm Hg diastolic
Table 2. Ambulatory heart rate and blood pressure by median ECG Cornell index
Characteristic
Cornell < 107.5 (n = 88)
Cornell ≥ 107.5 (n = 89)
All (n = 177)
P
Heart rate (bpm)
24-Hour
71.5 ± 7.8
73.3 ± 8.9
72.4 ± 8.4
0.16
Awake
77.4 ± 8.9
79.0 ± 11.1
78.3 ± 10.0
0.29
Asleep
60.2 ± 9.3
62.5 ± 9.8
61.4 ± 9.6
0.11
24-Hour
123.0 ± 8.8
126.4 ± 10.3
124.7 ± 9.7
Awake
127.5 ± 10.2
130.2 ± 10.8
128.9 ± 10.5
0.078
Asleep
115.0 ± 11.7
119.3 ± 12.9
117.2 ± 12.5
0.021
72.4 ± 7.7
75.3 ± 8.3
73.8 ± 8.1
0.016
Blood pressure
Peripheral systolic
0.020
Peripheral diastolic
24-Hour
Awake
77.3 ± 8.5
79.5 ± 8.7
78.4 ± 8.7
0.098
Asleep
63.1 ± 8.7
67.6 ± 10.6
65.4 ± 9.9
0.0026
50.7 ± 7.3
51.1 ± 8.5
50.9 ± 7.9
0.69
Peripheral pulse pressure
24-Hour
Awake
50.1 ± 8.6
50.8 ± 9.2
50.4 ± 8.9
0.64
Asleep
51.9 ± 8.7
51.8 ± 8.9
51.8 ± 8.8
0.92
111.3 ± 8.5
114.5 ± 9.8
112.9 ± 9.3
0.023
Central systolic
24-Hour
Awake
115.1 ± 9.4
117.3 ± 9.9
116.2 ± 9.7
0.13
Asleep
104.6 ± 11.4
109.2 ± 12.9
106.9 ± 12.4
0.012
73.9 ± 7.6
76.9 ± 8.3
75.4 ± 8.1
0.012
Central diastolic
24-Hour
Awake
79.1 ± 8.6
81.3 ± 8.8
80.2 ± 8.7
0.087
Asleep
64.3 ± 8.7
68.9 ± 10.6
66.6 ± 10.0
0.0019
37.4 ± 5.1
37.5 ± 6.2
37.5 ± 5.7
0.87
Central pulse pressure
24-Hour
Awake
36.0 ± 6.1
36.0 ± 6.7
36.0 ± 6.4
0.96
Asleep
40.3 ± 7.8
40.3 ± 7.8
40.3 ± 7.8
0.97
Values are mean ± SD. P indicates the significance of the difference between the participants with Cornell voltage <107.5 and ≥107.5mV·ms
(median). Abbreviation: ECG, electrocardiogram.
4 American Journal of Hypertension
ECG Voltage and Peripheral and Central BP
Figure 1. Diurnal profiles in 177 study participants of central systolic pressure (a), central diastolic pressure (b), central pulse pressure (c), and heart rate
(d). Plotted values are 2 hourly mean with 95% confidence interval. P values are for the comparison between awake and asleep averages.
(CI, 11.8–14.3; P < 0.0001), whereas peripheral PP slightly
increased (1.4, CI, 0.1–2.6; P = 0.029). Compared with BP
during waking hours (Table 2 and Figure 1), central BP during sleep decreased by 9.3 mm Hg systolic (CI, 7.7–10.9;
P < 0.0001) and 13.6 mm Hg diastolic (CI, 12.3–14.9;
P < 0.0001), whereas central PP increased by 4.3 mm Hg
(CI, 3.1–5.5; P < 0.0001). The awake–asleep decrease in systolic BP (11.7 vs. 9.3 mm Hg) was 2.4 mm Hg (CI, 1.7–3.1;
P < 0.0001) greater peripherally than centrally. In contrast,
the awake–asleep decrease in diastolic BP (13.1 vs. 13.6 mm
Hg) was 0.6 mm Hg (CI, 0.4–0.7; P < 0.0001) less peripherally than centrally. Consequently, the awake–asleep change
in PP (1.4 vs. 4.3 mm Hg) was 2.9 mm Hg (CI, 2.2–3.7 mm
Hg; P < 0.0001) less peripherally than centrally. Based on
comparison of the awake and asleep BP levels, there was in
all participants combined significant diurnal BP rhythmicity. When we applied the runs test to individual 24-hour
BP recording, among 177 workers, there was a significant
(P < 0.05) diurnal rhythm in 96 (54.2%) for systolic pressure,
in 104 (58.8%) for diastolic pressure, in 69 (39.0%) for PP,
and in 129 (72.9%) for heart rate.
Associations of ECG indexes with peripheral vs. central BP
The ECG indexes showed formally significant (P ≤ 0.05)
or borderline significant (P ≤ 0.07) positive associations
with 24-hour and asleep systolic and diastolic BP (Table 3).
In contrast, the ECG indexes were unrelated to central and
peripheral PP (P ≥ 0.29). Per 1-SD increment in systolic
pressure, the Cornell voltages were 0.104/0.086 mV and
0.082/0.105 mV higher in relation to brachial 24-hour and
asleep BP and 0.088/0.90 mV and 0.087/0.107 mV higher
in relation to central BP (Table 3). The corresponding
estimates for the Cornell index were 9.6/8.6 and 8.2/10.5
mV·ms peripherally and 8.6/8.9 and 8.8/10.7 mV·ms centrally. The regression slopes were similar for brachial and
central BP (P ≥ 0.067). These findings remained unchanged
if in the regression models we substituted peripheral pressure by its residual that removed the variance explained by
central pressure or vice versa. Figure 2 graphically displays
the regression slopes of the ECG indexes plotted against
24-hour peripheral and central systolic pressure. For clarity, the data markers in Figure 2 are averaged peripheral
and central systolic pressure by sixths of the distributions
of ECG indexes. Finally, Figure 3 shows the correlations
of the ECG indexes with 24-hour and asleep systolic and
diastolic pressures. P values derived by the Hotelling–
William test indicate that none of the pairwise differences
in the correlations of ECG indexes with peripheral and
central BP reached significance (P ≥ 0.088). Sensitivity
analyses adjusted for body mass index and skinfolds were
confirmatory.
DISCUSSION
The key findings of our current study can be summarized
as follows: (i) central systolic and diastolic BP and PP follow the same diurnal rhythm as brachial BP; (ii) the Cornell
voltage and Cornell index were positively and significantly
or borderline significantly associated with peripheral and
American Journal of Hypertension 5
Yang et al.
Table 3. Association of Cornell voltage and index with peripheral and central blood pressure
Cornell voltage (mV)
Cornell index (mV·ms)
Association size
Blood pressure (mm Hg)
Peripheral pressure
Association size
Central pressure
P
Peripheral pressure
Central pressure
P
Systolic pressure
24-Hour
0.104 (0.016 to 0.191)†
0.088 (0.0003 to 0.177)†
0.36
9.6 (0.65 to 18.6)†
8.6 (–0.40 to 17.6)*
0.54
Awake
0.086 (–0.001 to 0.175)*
0.062 (–0.026 to 0.151)
0.19
7.7 (–1.30 to 16.7)
5.8 (–3.2 to 14.8)
0.32
Asleep
0.082 (–0.006 to 0.170)*
0.087 (–0.001 to 0.175)*
0.74
8.2 (–0.82 to 17.2)*
8.8 (–0.22 to 17.7)*
0.68
24-Hour
0.086 (–0.002 to 0.174)*
0.090 (0.002 to 0.178)†
0.076
8.6 (–0.41 to 17.6)*
8.9 (–0.04 to 17.9)*
0.087
Awake
0.056 (–0.032 to 0.145)
0.060 (–0.029 to 0.149)
0.067
5.6 (–3.42 to 14.6)
6.0 (–3.1 to 15.0)
0.10
Asleep
0.105 (0.017 to 0.192)†
0.107 (0.019 to 0.194)†
0.45
10.5 (1.6 to 19.5)†
10.7 (1.8 to 19.6)†
0.55
24-Hour
0.040 (–0.049 to 0.129)
0.016 (–0.073 to 0.105)
0.24
3.2 (–6.0 to 12.1)
1.3 (–7.8 to 10.4)
0.38
Awake
0.048 (–0.041 to 0.137)
0.012 (–0.077 to 0.101)
0.12
3.6 (–5.4 to 12.7)
0.68 (–8.4 to 9.7)
0.20
Asleep
0.001 (–0.091 to 0.088)
0.001 (–0.087 to 0.090)
0.90
–0.29 (–9.4 to 8.8)
0.21 (–8.9 to 9.3)
0.82
Diastolic pressure
Pulse pressure
Estimates (95% confidence interval) reflect the association size per 1-SD increment in blood pressure (mm Hg). Significance of the association sizes: *P ≤ 0.07 and †P ≤ 0.05; P values are for the differences in association sizes between peripheral and central blood pressure
measurements.
Figure 2. The Cornell voltage (a) and index (b) plotted against peripheral and central systolic 24-hour blood pressure (SBP). The data markers are averages by sixths of the distributions of ECG indexes. The lines are the slopes of the ECG indexes on peripheral and central SBP averaged by sixths of the
distributions of the ECG indexes. Pperipheral and Pcentral indicate the corresponding significance levels. Pdifference is the significance of the difference between
the slopes for peripheral and central SBP. Abbreviation: BP, blood pressure; ECG, electrocardiogram; SBP, systolic blood pressure.
central BP over the whole day and during sleep; (iii) associations of Cornell voltage and Cornell index with BP were
not tighter for central compared with peripheral BP. The
association of the ECG indexes with the asleep, but not with
the awake BP, can be explained by the higher level of standardization of the nighttime recordings, when participants
were sleeping in the supine position and not exposed to the
physical and psychological stressors of work during daytime.
In our current study, brachial systolic pressure was 9.0 mm
Hg higher on awake ambulatory than office measurement,
because office BP was measured in a quiet environment in
the sitting position, whereas ambulatory monitoring was
performed on working days when the laborers were standing
and physically active along the production lines. The small
2.1-mm Hg differences in brachial office and awake diastolic
BP, which is well within validation criteria of ambulatory
6 American Journal of Hypertension
devices,24 can probably be explained by using the auscultatory vs. oscillometric approach.
To our knowledge, few previous studies25,26 assessed the
central BP in ambulatory conditions. In the population-based
Genotipo, Fenotipo y Ambiente de la Hipertensión Arterial
en Uruguay Study (GEFA-HT-UY),26 investigators applied
the same technology as in our current study. This population sample included 167 participants (mean age, 56.1 years;
63.5% women).26 The Ambulatory Central Aortic Pressure
(AmCAP) study described the diurnal patterns of simultaneously measured 24-hour ambulatory brachial and central BPs in 171 hypertensive patients (mean age, 53.6 years;
53.2% women) enrolled into the ASSERTIVE trial.25 The
brachial and central pressures were measured by an oscillometric and tonometric approach, using the SpaceLabs
monitor (Spacelabs Healthcare, Snoqualmie, WA) and the
ECG Voltage and Peripheral and Central BP
Figure 3. Correlations of Cornell voltage (a) and index (b) with 24-hour and asleep systolic and diastolic pressures. Data markers and whiskers represent
the point estimates of the correlation coefficients and their 95% confidence interval, respectively. P values derived by the Hotelling–William test denote
the significance of the pairwise comparison of peripheral (open symbols) vs. central (closed symbols) blood pressure. Abbreviation: BP, blood pressure.
BPro wrist device (HealthSTATS International, Singapore),
respectively.25 In GEFA-HT, daytime was the interval from
10 am until 8 pm and nighttime ranged from midnight to 6
am. These fixed intervals eliminate the transition periods in
the morning and evening when BP changes rapidly, resulting
in daytime and nighttime BP levels that are within 1–2 mm
Hg of the awake and asleep levels.27 In AmCAP,25 these transition periods were not excluded from analysis and daytime
ranged from 6 am until 10 pm and nighttime from 10 pm
until 6 am. In spite of these methodological differences—in
line with our current findings in workers—both studies25,26
demonstrated a high degree of parallelism between the diurnal course of peripheral and central BP.
We searched PubMed for relevant publications without
limitations of publication date or language using as terms
“central blood pressure” OR “central BP” OR “ambulatory
blood pressure” OR “ambulatory BP” OR “24-hour blood
pressure” OR “24-hour BP” AND “ECG” OR “ECG voltage” OR “left ventricular hypertrophy” OR “hypertrophy”
OR “electrocardiography”. Our literature search revealed
only two other studies with possible relevance to the issue
addressed in the current manuscript.28,29 In 728 participants
(57.6% women) enrolled in the Czech post-MONICA study
(Monitoring Trends and Determinants in Cardiovascular
Disease), Wohlfahrt and colleagues assessed the Sokolow–
Lyon index and central BP determined by a static tonometric approach (SphygmoCor, Atcor Medical Ltd, West
Ryde, Australia).28 The prevalence of electrocardiographic
left ventricular hypertrophy was only 9.4% (n = 17) among
181 participants younger than 45 years and 9.0% (n = 43)
in 547 older participants. In the younger participants, the
standardized regression coefficients relating the Sokolow–
Lyon index to BP with adjustments applied for sex and body
mass index were 0.04 mV/mm Hg (P = 0.56) vs. 0.10 mV/
mm Hg (P = 0.15) for peripheral vs. central systolic BP and
0.09 mV/mm Hg (P = 0.23) vs. 0.10 mV/mm Hg (P = 0.20)
for peripheral vs. central PP. The Czech authors recognized
that there was a problem of collinearity but did not formally
compare the estimates produced by peripheral vs. central
BP in the younger participants. In the older participants,
with adjustments applied for sex age, heart rate, and use
of antihypertensive drugs (30.7%), the standardized odds
ratios relating left ventricular hypertrophy to BP were 1.046
vs. 1.113 for peripheral vs. central systolic BP and 1.034 vs.
1.101 for peripheral vs. central PP. All odds ratios were significant (P < 0.001).28 As a work-around to avoid the problem of collinearity, Wohlfahrt and coworkers reported that
in older participants the area under the curve for discriminating electrocardiographic left ventricular hypertrophy was
0.90 vs. 0.83 for central vs. peripheral systolic BP (P < 0.05)
and 0.90 vs. 0.81 for central vs. peripheral PP (P < 0.05).28
They concluded that the noninvasively determined central
pressure in older individuals was more strongly related
to electrocardiographic left ventricular hypertrophy than
brachial pressure, but that in younger subjects the voltage
criteria of left ventricular hypertrophy were not independently associated with central and brachial BP.28 The interpretation of the Czech report28 is not straightforward, as
the 45-year age threshold is arbitrary and about one third
of the older participants were on antihypertensive drug
treatment. Measurements of central BP in the Czech study28
were momentary and did not cover the whole day as in our
present study. Furthermore, Gómez-Marcos and colleagues
enrolled 1,544 patients (mean age, 55 years; 61% women)
recruited from primary care into the EVIDENT cross-sectional observational study (Physical Exercise, Fitness and
Dietary Pattern and Their Relationship with Blood Pressure
Circadian Pattern, Augmentation Index and Endothelial
Dysfunction Biological Markers; NCT01083082).29
Electrocardiographic left ventricular hypertrophy was associated with the 24-hour, awake, and asleep systolic BP, but
the authors did not formally compare the associations with
American Journal of Hypertension 7
Yang et al.
peripheral vs. central BP.29 In summary, what our study
adds to the current literature28,29 is (i) the recruitment of
participants at a stage in life when the association between
ECG voltages and BP can already be picked up, but when
the prevalence of left ventricular hypertrophy in response to
the BP load is still low; (ii) measurement of central BP over
the whole day rather than momentary as in other studies;
(iii) and the proper statistical approach to account for the
collinearity between peripheral and central BP. The clinical implication is that given the timeframe over which left
ventricular hypertrophy develops, early intervention with
hypertension is a prerequisite to prevent cardiovascular
complications.
Our literature search revealed two other studies that
focused on echocardiographic left ventricular mass index30
or hypertrophy6,30 in relation to the 24-hour brachial and
central BP measured by the same technology as implemented in the present study. However, these 2 studies6,30
produced contradictory results. Protogerou and coworkers
showed in 229 patients (mean age, 54.3 years; 43% women),
of whom 75% were hypertensive, that 24-hour central systolic BP was significantly better associated with left ventricular mass index and left ventricular hypertrophy than
the 24-hour and office brachial systolic BP, independent of
sex, age, obesity, and antihypertensive drug treatment.30 As
in the Czech ECG study,28 receiver operator characteristics
curves showed a higher discriminatory ability of 24-hour
central than brachial systolic BP to detect the presence of left
ventricular hypertrophy (area under the curve, 0.73 vs. 0.69;
P = 0.007). de la Sierra and coworkers enrolled 208 hypertensive patients, of whom 37.0% had echocardiographic left
ventricular hypertrophy.6 With adjustments applied for sex,
age, and antihypertensive drug treatment, the odds ratios
expressing the risk of target organ damage per mm Hg,
including left ventricular hypertrophy, were 1.056 vs. 1.053
for peripheral vs. central systolic BP and 1.076 vs. 1.081 for
peripheral vs. central PP.6 When introduced in the same
logistic model only peripheral—not central—BP retained
significance.6 Two additional studies31,32 assessed the association of left ventricular mass index31,32 or left ventricular
hypertrophy31 with central BP measured in the supine position using a tonometric approach. Among 2,585 participants
enrolled in the Strong Heart Study31 (mean age, 40 years;
60% women), the unadjusted correlations coefficients relating left ventricular end-diastolic diameter to BP were closer
for brachial than central systolic BP (0.242 vs. 0.179) and PP
(0.165 vs. 0.135). The opposite was observed in relation to
relative wall thickness and left ventricular mass index. For
relative wall thickness, the correlation coefficients in relation
to peripheral and central systolic BP and PP were 0.250 vs.
0.286 and 0.130 vs. 0.167. The corresponding estimates for
left ventricular mass index were 0.374 vs. 0.396 and 0.290
vs. 0.335. In view of the large sample size,31 these marginal
but inconsistent differences reached formal statistical significance. In a study of 392 treatment-naïve hypertensive
patients (mean age, 49 years; 45% women),32 the unadjusted
correlation coefficients relating left ventricular mass index
to BP were similar for office and the tonometrically assessed
central systolic pressure (0.21 vs. 0.19).
8 American Journal of Hypertension
Strong points of our current study are that we measured
central BP under ambulatory—not static—conditions, that
we report on quality control of the ambulatory recordings
based on the number of peripheral and central BP readings available for analysis, that participants kept a diary, the
gold standard33 to document the awake and asleep portions
of the day, that all ambulatory BP readings in individual
recordings were processed using the same standardized SAS
macro, and that the initial participation rate was as high as
68.6%. On the other hand, our study must also be interpreted within the context of its potential limitations. First,
we excluded 100 potentially eligible workers, because of
the quality of the ambulatory central hemodynamic readings. However, workers analyzed and excluded had similar
age (29.1 vs. 28.3 years), body mass index (28.1 vs. 29.6 kg/
m2), systolic/diastolic brachial BP in office (120.0/80.5 vs.
120.3/80.9 mm Hg), and 24-hour ambulatory (124.7/73.8 vs.
124.4/74.1 mm Hg) measurement and Cornell index (114.8
vs. 121.6 mV·ms). Second, the median number of ambulatory readings was only 35 over a whole day, because participants, most of whom were production line workers doing
physically strenuous labor, had the option to cancel readings interfering with their work rhythm. Third, the sample
size was relatively small, but nevertheless of the same order
of magnitude as in other reports.25,26 Of note, studies with
an ECG28,29 or echocardiographic6,30–32 outcome related to
peripheral and central BP with sample size ranging from
2086 to 2,58531 produced contradictory results. Fourth, we
conducted our study in predominantly young men enrolled
in the work force of lead acid battery manufacturing and
recycling plants in the United States. Our main finding that
there is no difference in the associations of the ECG indexes
with peripheral and central BP should therefore not be
extrapolated to women, older men or the general population. Finally, the prevalence left ventricular hypertrophy
among the workers was only 1 (0.6%) or 6 (3.4%) by Cornell
voltage or index criteria, precluding any categorical analysis of the ECG indexes. However, the spread of the Cornell
voltage (5th to 95th percentile interval, 0.29–2.34 mV) and
the Cornell index (25.5–230.4 mV·ms) was wide and cannot
explain absence of any difference in the associations of the
ECG indexes with peripheral vs. central BP.
Perspectives
Whether or not central BP is more closely related to target organ damage or is a better predictor of adverse health
outcomes remains a matter of debate. Opinions range from
the view point that central BP is an independent predictors of future cardiovascular events and all-cause mortality34–36 to that there is no compelling scientific or practical
reason to replace brachial systolic BP with any of the newer
hemodynamic measures in the vast majority of clinical situations.17,34 Only clinical trials, in which patients would be
randomly allocated to interventions specifically lowering
central BP37 vs. no intervention can definitely resolve the
debate. Previous experience38 shows that ECG voltages and
ECG criteria for left ventricular hypertrophy might be used
as study endpoints in such trials, because these intermediate
ECG Voltage and Peripheral and Central BP
outcomes can be reached within 6 months of randomization.
ECG voltage indexes39 and left ventricular hypertrophy40,41
are strong and independent predictors of adverse cardiovascular outcomes. From a clinical perspective, our study
does not support any incremental value of central over and
beyond brachial BP in risk stratification.
ACKNOWLEDGMENTS
The authors acknowledge the expert clerical assistance
of Vera De Leebeeck, Yvette Piccart, and Renilde Wolfs.
ILZRO supports SPHERL by an unrestricted research grant.
The European Union (HEALTH-F7-305507 HOMAGE)
and the European Research Council (Advanced Researcher
Grant 2011-294713-EPLORE and Proof-of-Concept Grant
713601-uPROPHET) and the Fonds voor Wetenschappelijk
Onderzoek Vlaanderen, Ministry of the Flemish Community,
Brussels, Belgium (G.0881.13) currently support the Studies
Coordinating Centre in Leuven.
DISCLOSURE
The authors declared no conflict of interest.
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